In conventional microphones, a diaphragm ring, a diaphragm, a spacer, a back electrode, a holder, a gate ring, and a substrate, for example, are stacked in a cylindrical metal capsule having sound apertures and the components are fixed by caulking the end of the capsule toward the substrate (Japanese Patent Application Laid Open No. 2003-153392 (Patent Reference 1)). Electrodes are protruded from the substrate for conduction of electricity with an external object. The caulked part has a rounded portion (prominent portion) and the extent to which the portion is rounded (the height of the prominence) varies. That is, the amount of the protrusion of the electrodes with respect to the caulked part varies. Therefore, when such a microphone is soldered using a reflow furnace, the unevenness causes poor soldering in the reflow furnace or a faulty posture (tilt) of the microphone mounted on a wiring board.
To solve the problem, the applicant has previously proposed a structure in which the disposition of components in the cylindrical metal capsule is reversed (Japanese Patent Application No. 2005-121051 filed on Apr. 19, 2005). FIG. 1 shows a cross-sectional view of the microphone previously proposed by the applicant. According to the related art, a ground electrode pattern 114 is formed on the side (bottom 121) in which opening 123 of a capsule 102 is provided. A built-in substrate 112 is provided on the ground electrode pattern 114. The built-in substrate 112 has an output terminal electrode 111 and ground terminal electrode 115 on the same side on which the ground electrode 114 is provided. The terminal electrodes 111, 115 are longer than the thickness of the capsule 102 and protrude outward through the opening 123 of the capsule 102. A conductor pattern 109 is formed on the upper surface of the built-in substrate 112 and an electronic circuit 110 is provided on it. Stacked on the upper surface of the built-in substrate 112 are a gate ring 108, a holder 107, a back electrode 106, a spacer 105, a diaphragm 104, a diaphragm ring 103, and a top plate 130 having sound apertures 131. The end of the capsule is caulked to the top plate 130, thereby fixing each of the components as well. The top plate 130 may be made of the same metal as the capsule 102 and may have the same thickness as the capsule 102, for example.
In this microphone 100, the terminal electrodes 111, 115 can be reliably protruded with respect to the thickness of the bottom 121 without being affected by unevenness of the caulked part 113. Accordingly, defects in soldering using a reflow furnace can be prevented.
However, for example, if the microphone 100 is installed in a cell phone, the microphone 100 picks up touch noise generated when a user touches the cell phone, vibration noise generated by driving of a built-in motor and the like. This problem is unavoidable as long as the microphone is directly mounted on a wiring board.
FIG. 2 shows a circuit configuration of an analog microphone. Contained in a capsule 102 are an acoustic-electric converter 100′ and an electronic circuit 110. The acoustic-electric converter 100′ is formed by the capsule 102 and internal components. The electronic circuit 110 consists of a field-effect transistor (FET) and a capacitor, for example. As can be seen from FIG. 2, the microphone 100 has two terminals: an output terminal and a ground terminal. It should be noted that, the terminal electrode (ground) 115 is shown in two positions in FIG. 1 because FIG. 1 is a cross-sectional view of a toroidal terminal.
The applicant has also proposed previously, in another application, an electret condenser microphone that can be soldered using a reflow furnace and outputs a digital signal (Japanese Patent Application No. 2005-320815 filed on Nov. 14, 2005). FIG. 3 is a cross-sectional view of an exemplary electret condenser microphone outputting a digital signal proposed by the present applicant. The front type electret condenser microphone 200 has an electret polymer film made of a heat-resistant material within an electrically conductive capsule 201. An electrically conductive diaphragm 207, an electrically conductive ring 208, a gate ring 209, and a wiring substrate 202 are provided and are separated from the electret polymer film by a spacer 206 made of an heat-resistant insulator. The end of the electrically conductive capsule 201 is caulked to the wiring substrate 202 and fixes the internal components. An IC device 210 is mounted on the interior side of the wiring substrate 202. Four terminals 204(a-d) are provided on the exterior side of the wiring substrate 202. The terminals 204(a-d) are protruded through an opening 223 of the front type electret condenser microphone 200 for conduction of electricity with an external object. With this configuration, a digital electret condenser microphone capable of resisting high temperatures generated by soldering in a reflow furnace can be implemented.
FIG. 4 shows a circuit configuration of a digital microphone. Provided in an electrically conductive capsule 201 are an acoustic-electric converter 200′ and an IC device 210. The acoustic-electric converter 200′ is formed by the capsule 201 and internal components. The IC device 210 includes an impedance converter/amplifier 210a and a digital sigma modulator 210b. As can be seen from FIG. 4, four terminals, a power supply terminal 204a, a clock input terminal 204b, a digital data output terminal 204c, and a ground terminal 204d, are provided. A problem with this digital microphone is that it is susceptible to high-frequency noise from nearby components because its ground terminal does not have a toroidal shape.
An approach to reducing the number of components of both analog and digital microphones may be to solder the bottom of the capsule directly to a wiring board, thereby omitting the ground terminal. In this case, if a ground electrode can be formed into a toroidal shape, the microphone would be less susceptible to high-frequency noise. However, some measures must be taken against heat transferred to the interior of the microphone during soldering in a reflow furnace. Furthermore, the vibration pickup problem cannot be solved by using the bottom itself as the ground electrode.